PET is a thermoplastic polyester that can be formed from 1,2-dihydroxyethane ("ethanediol") and terephthalic acid by direct esterification to form bis (hydroxy ethyl) terephthalate ester ("BHT") which is then polymerised by catalysed ester exchange to useful polymers.
Traditionally, PET has been used extensively because it can be offered as an oriented film or fibre, has high tenacity, good electrical resistance and low moisture absorption together with a melting point of approximately 265 degrees Celsius.
For these reasons, its uses have been very diverse extending from being blended with cotton for wash and wear fabrics, blended with wool for worsteds and suitings, packaging films and recording tapes and containers including soft drink containers.
There are a number of applications of PET where remelting and reforming is not permissible or gives inferior properties. The reuse of PET for these applications is best achieved by degrading the polymer into the original monomers namely ethanediol and terephthalic acid then reacting the monomers together to regenerate the original PET.
The known art (British Patent No. 610135) is to hydrolyse PET with either strong alkalis or acids. The alkalis must be neutralised with acid to produce terephthalic acid resulting in a significant cost of reagents and possible contamination of the product with alkali metal ions. The strong acids must be recovered and the ethanediol separated. Further the acids dissolve paper and pigments to give by products which are difficult to separate.
It is known that PET will transesterify with ethanediol (British Patent No. 610136) but the product BHT, is not a convenient intermediate for subsequent purification to remove esters of contaminant acids such as benzoic and adipic acid.
Accordingly, the present invention is directed to an improved process of decontaminating PET.